This disclosure relates to ballasts for powering fluorescent lamps including compact fluorescent lamps (CFLs). This type of lamp includes cathodes (filaments) which are preferably preheated before ignition to extend the operational life of the lamp. The lamp cathodes are covered with emission mix to facilitate passage of electrons through the gas for production of light. Over time, the emission mix is sputtered off of the cathodes in normal operation, but a larger amount is sputtered off when the lamp is ignited with cold cathodes. When the emission mix becomes depleted, a higher voltage is required for the cathodes to emit electrons, a condition sometimes referred to as end-of-life (“EOL”). The higher voltage results in an increase in temperature which may overheat the lamp and in some cases crack the glass if the lamp is not replaced.
Conventional low cost CFL ballasts often use a positive temperature coefficient (PTC) thermistor to heat the lamp cathodes of the lamp prior to ignition (preheat). The PTC is coupled in parallel with a capacitor connected across the CFL, and initially conducts allowing preheating current to flow through the lamp cathodes. With continued conduction, the PTC device heats up and the PTC resistance increases, eventually triggering ignition of the gas in the lamp. The PTC, moreover, is typically situated in close proximity to the lamp to keep the PTC in the high-impedance condition during normal operation of the lamp. However, PTC devices are costly and occupy valuable space in the ballast. In addition, the PTC element never reaches infinite impedance and thus conducts some amount of current throughout operation of the ballast (even if some of the energy to keep the PTC device warm comes from lamp heating). Thus, the use of PTC devices for cathode preheating negatively impacts ballast efficiency. Furthermore, PTC preheating circuits need time to cool before reapplication of power to avoid cold-cathode ignition and the associated lamp degradation. Thus, a need remains for improved ballasts and techniques for preheating fluorescent lamp cathodes without using PTC components.